1. Field of the Invention
This invention relates to a real-image type viewfinder, used in a photographing camera, a video camera and so on, and, in particular, to a real-image type viewfinder for a camera, such as a lens-shutter type camera, which has a viewfinder optical system independent of a photographing optical system.
2. Description of the Related Art
In general, a viewfinder optical system for a real-image type viewfinder includes: a positive objective lens system; an erecting optical system for converting an inverted finder-image, obtained from the positive objective lens system, into an erected finder-image; and an ocular lens system for viewing the erected finder-image therethrough.
As is well known, it is necessary to display various information patterns, such as a viewing-field-frame for defining a frame area to be photographed, a photometry-sensing-frame for defining a photometric area to be measured, suitable information symbols and so on, on a viewing field of the viewfinder. Namely, the various information patterns must be superimposed on the finder-image, viewed through the ocular optical system of the viewfinder optical system.
Conventionally, in order to superimpose the information patterns on the finder image, a transparent glass plate, on which the information-patterns are printed as opaque areas, is placed in the vicinity of an image plane of the positive objective lens system of the viewfinder optical system. Thus, when viewing through the ocular optical system, the finder-image, on which the information patterns are superimposed, can be observed, due to the transparent glass plate with the information patterns being located at the image plane.
The locating of the transparent glass plate in the viewfinder optical system is relatively easy and inexpensive. Nevertheless, the method utilized is unadvisable, as dust, dirt and other types of particulate matter may be easily deposited on the transparent glass plate. Of course, the adhesion of dust and dirt to the transparent glass plate impairs the visual clarity due to the transparent glass plate being positioned on the image plane.
Unexamined Japanese Patent Publication No. 1-113736discloses another approach for superimposing the information patterns on the finder-image. According to this approach, an optical information-pattern-forming system is incorporated into the viewfinder optical system, and includes a half mirror interposed between the erecting optical system and the ocular optical system, and a translucent plate, such as a frosted glass plate, having the information patterns printed as transparent-area segments thereon.
The translucent plate is positioned at a location that is the optical equivalent of the location of the image plane of the positive objective optical system. The translucent plate is illuminated with natural or external light, in such a manner that light rays, passing through the translucent plate, are directed to the ocular optical system through the half mirror. Thus, when viewing through the ocular optical system, the finder-image, obtained through the objective optical system, can be observed together with the information patterns superimposed thereon, which are obtained through the translucent plate.
As mentioned above, the information patterns are printed as transparent-area segments on the translucent plate. Therefore, providing particulate matter is only adhered to the translucent area of the translucent plate, the visual clarity of the ocular optical system is not impaired.
Nevertheless, the approach, disclosed in the above-mentioned publication, has some disadvantages as follows:
Firstly, an additional window, for introducing natural or external light into the optical information-pattern-forming system to illuminate the translucent plate, must be provided in a camera body, and thus a freedom of design of the camera body is restricted, due to the provision of the additional window.
Secondly, a focal length of the ocular optical system is increased, due to the existence of the half mirror interposed between the erecting optical system and the ocular optical system, resulting in a bulkiness of the viewfinder optical system.
Thirdly, when the concept of the previously-mentioned publication is applied to an existing camera, a basic design for a viewfinder optical system of the existing camera must be modified, due to the fact that the half mirror must be interposed between the erecting optical system and the ocular optical system. Of course, a modification to the basic design of the viewfinder optical system is very costly.